StandaloneMmPkg: Fix issue about SpPcpuSharedBufSize field

StandaloneMmPkg: Fix issue about SpPcpuSharedBufSize field

TF-A: TrustedFirmware-A
SPM: Secure Partition Manager(MM)

In TF-A, the name of this field is sp_shared_buf_size. This field is
the size of range for transmit data from TF-A to standaloneMM when
SPM enable.

SpPcpuSharedBufSize is pass from TF-A while StandaloneMM initialize.
So, SpPcpuSharedBufSize should be rename to SpSharedBufSize and this field
should no multiply by PayloadBootInfo->NumCpus;

Signed-off-by: Ming Huang <huangming@linux.alibaba.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>